New Geothermal Heat Pump Install & Driveway Heat

I am in the process of having a vertical loop geothermal system installed on new construction and thought that I might look into adding additional loop footage in sidewalks and driveways for snow melt. I understand that this is a different type system than the one for heating the home. My question is what type of footage do I need to handle 5000-6000 sq.ft. of concrete? Thanks in advance for your response.

Snowmelt application need a large amount of energy for a relatively short period of time, but you are only paying for the consumption when they are running during this short period. Geo systems are typically sized to delivery the right amount of heat relatively constantly and save you a lot of money proportionally to their running/usage time.

In other words, the economics are usually not there, when you need to build up a large capacity for a geo system which does not turn on much due to its application (snow melt). If you have access to natural gas, get a boiler installed for the snowmelt part.

To effectively melt snow, you are going to need AT LEAST 100 BTU/h per square foot (depending on your geographic location and class of snowmelt system). With the square footage you are saying, you can figure on 500,000 to 600,000 BTU/h's (plus pick up BTU/hs) in addition to your home's heatloss. The main drawback is that a geothermal unit simply cannot supply a high enough temperature to give you an adequate response time (time it takes from a cold start until system actually starts melting snow) unless you "idle" the slab at a few degrees below freezing.
Also, with these low supply water temperatures to be used with snowmelt, you will be using twice as much tubing in the slab as you would if you were able to utilize 130 to 140 degree supply water temperature.
If you want to use geothermal to snowmelt 5,000 - 6,000 square feet of driveway, install a 70 ton water to water unit, put 10,000' to 12,000' of 3/4" PEX at 6" O.C. , and idle the slab at 28 degrees F.
I personally agree with the response of letting a boiler handle the snowmelt load.

It has been awhile since I've looked into this but I think Dan's btuh / SF value may be a bit high.

One of the standard engineering handbooks (possibly ASHRAE Fundamentals) has a table for this app based on geography and class of service (residential, commercial and hospital) resi loads are much lower, the thinking being the system's performance isn't quite as critical as that needed for an ambulance approaching an ER.

If this is new construction look into subslab insulation - that way melt heat goes only up, not into ground

Electric mats are an option, but beware high cost of use, especially in a demand or Time-of-Day metering option. Still, they might make sense in that installed cost is lower, they are simpler, not vulnerable to freezing, and may not be needed all that often.

I quite agree that this is likely a poor app for geo given the high load relative to house load. It MIGHT be able to be made to work with a thermal accumulator (insulated storage tank).

In many areas the need for deicing does NOT coincide with design low temperatures. In other words, outdoor temp during an deicing event may be enough above design temp such that a ton or two of spare capacity is available. The only way for that to be effective, though, would be to store heat from that extra capacity in advance of the deicing need. A downside risk is that very cold weather often follows snow, and the loopfield might be drawn down by deicing ahead of bitter cold weather.

Best case, such an arrangement might be a Fankensystem that only an engineer could love...

I doubt it also Mark; however, I'm thinking it would possibly do the job if you tweaked some things...
If you put more piping in the concrete loop like 1/2" on 4" or 6" centers and had a good amount of volume in your ground loop that you could potentially just let the pump keep the slab above 35 degrees at all times kind of thing? You might need a supplemental heat on really cold days?

I doubt it also Mark; however, I'm thinking it would possibly do the job if you tweaked some things...
If you put more piping in the concrete loop like 1/2" on 4" or 6" centers and had a good amount of volume in your ground loop that you could potentially just let the pump keep the slab above 35 degrees at all times kind of thing? You might need a supplemental heat on really cold days?

Click to expand...

It is not the density of the pipe which is the issue with snow melt, it is the fact that forcing water through the phase change from solid to liquid consumes a large amount of energy. So heating a pound of frozen water ice from 31F to 32F consumes 1 BTU, but it consumes 144 BTUs to go from 32 to 33F and melt the water.
You need something which delivers a lot of energy in a short period of time. Geo delivers a constant energy over a log period of time, kind of the opposite you need for snowmelt. In other words, you need a lot of energy and a large system capacity, which you only need for a short period of time. Again, not something geo does well.

The thinking is a bit off in this forum. I have limited snow melt experience, but have attended 2 lectures on it. I can dig up the specifics if required.

You melt snow as it lands. So predictive weather model controls are almost a necessity. And pre-heating the slab/driveway does not require huge loads, but it is definitely a large energy draw.

We do skirt melts and use on demand gas/propane systems for these purposes. But only to simplify the process of a very high glycol system outdoors vs an indoor system. Simplifying the mechanics. But I see no issue with a true dedicated geothermal snow melt system.

Thermal mass is still the same. Still takes 144 BTUs to melt one pound of ice which is 32F cold, versus 1 BTU to heat the pound of water up by another degree. Thanks for reminding us of this, in this case very insignificant, detail.

I have been daydreaming of using geo thermal for snow melt for quite some time now. I live in S.E. WI and have had my fill of dealing with the white stuff. I do not posses the engineering skills and knowledge to make it happen though. That's why I feel fortunate to have stumbled across this forum. It seems to be full of knowledge and experience. (flattery, anyone?) I like Adm. Cole's idea of idling the slab at ground temp, but I don't want to run a loop to Yellowstone. (Mark C) My idea was to drill many smaller holes, example, 12" d x 20' deep at some spacing, then insert a re-bar cage with some amount of pipe coiled around the base like threads on a bolt. Then fill the holes with lean mix concrete, run the pipes in the slab at some spacing and pour it. All I have to do then is hook up the pump and walk away. It would be one continuous loop. The fluid goes down, gathers heat, comes up warms the slab and goes back down again for another round of heat. Couldn't it be that simple? I would like to believe so. The driveway would last forever, never freezing and never heaving being anchored by what essentially would be caissons.